Device for causing an untuned structure to vibrate ultrasonically

ABSTRACT

The present invention relates to the vibration of an untuned structure by means of an ultrasound converter. The main but not unique purpose is to cause a filtering cloth, a perforated metal sheet or a screening mesh to vibrate in order to improve the flow rate in the filtering or sieving process, with equal surface energy, without clogging the mesh or harming the product being processed. The device according to the invention is characterized in that it comprises at least one electro-acoustic converter (12) ridigly fixed to said structure (10) by means of metallic securing elements (14) tuned to the frequency of the converter, the links with the structure being in a maximum region of amplitude (V)j of said securing elements and resonance of the assembly being provided by a nut or any metallic unit tuned as an integral multiple of a half wave length.

The present invention concerns a device for causing one or more untunedstructures to vibrate ultrasonically. These structures may be of verydifferent types, but the present description will be essentially limitedto a screening structure, although this must not be interpreted aslimiting the present invention.

It has been known for many years that causing a screen to vibrateultrasonically effects a qualitative and quantitative improvement in thescreening of fine powders or fine materials considered difficult toprocess.

Consider for example the problems encountered with mineral, metal andceramic powders, with particle sizes in the order of one micron andless, and with spheroidal granules which tend to clog the screens.Ultrasonic techniques have already been described in the literature ofthis art and in particular in the work entitled Crowford Engineering ofFrederick and Co.

The state of the art is also shown by FR-A-2 233 108 which describesscreening equipment provided with an ultrasonic transducer in directcontact with the filter cloth.

Note that a device of this kind is restricted to circular screens. Asultrasound is emitted from a single anchor point the mesh is subject tonon-negligible stresses at this level which may cause premature wear andconsequential pollution of the product treated.

In the case of a multistage screen the number of transducers must bemultiplied commensurately and this complicates the construction. Thisprior art device has another drawback in that, in all cases, theconverter is exposed to direct contact with the treated product and mustusually be sealed.

The document DE-A-38 13 178 discloses a device for causing to vibrate atan ultrasonic frequency a structure comprising at least oneelectro-acoustic converter adapted to vibrate in a given vibrationdirection and fixed rigidly to said structure by means of metal fixingmembers tuned to the converter frequency, the links with the structurebeing in a region of maximum amplitude of said fixing members.

An object of the present invention is to provide an ultrasonic vibratordevice which circumvents all the drawbacks of the prior art devices.

The main aim of the present invention is to render a support structureresonant. This is, for example, a filter cloth, membrane or other platesupport which is not resonant in itself at the frequency of theultrasonic emitter.

Another aim of the ultrasonic device in accordance with the invention isto optimize the ultrasonic efficiency regardless of the shape and sizeof the structure to be caused to vibrate. Naturally a main objective ofthe present invention is to prevent direct contact of the ultrasonictransducer with the filter cloth or membrane. In the case of a screen,the device in accordance with the invention excites the mesh by means ofultrasonic energy previously distributed in its supporting frame, sominimizing stresses at the anchor points. Finally, as explained in moredetail later in this description, the ultrasonic device in accordancewith the invention may be fitted to existing structures by modifying thegeometrical shape of coupling members tuned to integer multiples of thehalf-wavelength at the output frequency.

In accordance with the present invention these objectives are achievedby a device for causing an untuned structure to vibrate at an ultrasonicfrequency which is characterized in that it comprises at least oneelectro-acoustic converter fixed rigidly to said structure by means ofmetal fixing members tuned to the frequency of the converter, thestructural couplings being located in a maximum amplitude region of saidfixing members and the assembly being made resonant by fixing a nut orany other metal assembly tuned to an integer multiple of thehalf-wavelength.

Other features and advantages of the present invention will emerge fromthe following detailed description with particular reference toembodiments shown in the appended drawings in which:

FIGS. 1 and 2 show two different types of coupling bar profile formatching dimensional constraints of a structure to be caused to vibratewhilst achieving the required resonant frequency;

FIGS. 3 through 8 show various embodiments for causing single ormultiple structures of various shapes to vibrate ultrasonically.

It will be remembered that one essential aim of the ultrasonic device inaccordance with the invention is to be readily adaptable to differenttypes of already existing structures and to structures having shapes andsizes dependent on their application. The ultrasonic device inaccordance with the invention achieves this objective in the twoembodiments diagrammatically represented in FIGS. 1 and 2.

In the figures this structure 10 is shown schematically. It is anyuntuned structure, for example a structure supporting any operative unitsuch as a filter cloth.

The ultrasonic device in accordance with the invention comprises anelectro-acoustic converter 12 which must be fixed rigidly to saidstructure 10. In all the embodiments described the links with thestructure 10 are in a region of maximum amplitude V.

Fixing is by means of metal members which are tuned to the frequency ofthe converter, the length of said members including the thickness of thelugs joining them to the structure 10. In the embodiments shown in FIGS.1 and 2 the metal fixing members are coupling bars 14 disposed betweentwo facing walls of the structure 10.

These metal fixing members must naturally be tuned to the frequency ofthe converter. In this instance their length is half the wavelength atthe output frequency of the ultrasonic converter, resonance of theassembly requiring the presence of a nut or any other metal member tunedto the operating frequency extending the coupling members. Theultrasonic converter may comprise one or more emitters of any kind, forexample electrostrictive, magnetostrictive, electrocapacitative orpiezo-electric emitters.

Note that the coupling bars have an exterior surface whose profilematches the specific dimensional configuration of the structure and iscompatible with the required frequency of resonance.

The ultrasonic device of FIG. 1 illustrates one manner of operating atthe half-wavelength at the frequency of the converter 12. In the FIG. 1embodiment the exterior surface of the coupling bars 14 has, in thevicinity of their nodal area, a radial contraction 16 which has symmetryof revolution about the axis of said bars 14.

This specific embodiment, in which the coupling bars have a contractionin the vicinity of their nodal zone because of the presence of a groove16 or the like, reduces the size of the coupling bars 14 without thisbecoming incompatible with resonance.

On the other hand, the embodiment shown in FIG. 2 employs coupling bars14 whose exterior surface has, in the vicinity of their nodal zone, aradial enlargement 18 which also has symmetry of revolution about theaxis of the bars 14.

The specific profile adopted in the FIG. 2 embodiment enables the lengthof the coupling bars 14 to be increased in a manner that is compatiblewith the frequency of the converter 12.

FIG. 3 shows how a support frame 10 holding a filter cloth 20 is causedto vibrate. The vibrator device is fixed to two opposite sides 22, 24 ofthe support frame 10. In this configuration the electro-acoustic devicedeployed comprises a unidirectional converter 12, a coupling bar 26which is tuned to an integer multiple of half the wavelength and a nut28 which is also tuned to half the wavelength. Note that the vibratorassembly is fixed to the support frame 10 in a region of maximumamplitude. The various members, namely the converter 12, the couplingbar 26 and the tuned nut 28, are advantageously screwed together throughholes in facing parts of the support 10. Note, however, that thevibrator device may be rigidly fastened to the structure 10 by any otherappropriate means, the essential requirement being to obtain a totallyrigid coupling between the structure and the vibrator device. Thevarious units may be force-fitted together, or adhesively bonded and/orwelded together, for example.

In the FIG. 3 embodiment the holes receiving the screwthreads couplingtogether the converter 12, the coupling bar 26 and the tuned nut 28 maybe replaced by slots opening onto the upper edge of the members 22 ofthe support frame. The areas of reduced cross-section where the variousmembers 12, 26 and 24 are coupled together may be force-fitted into saidslots to provide the rigid coupling to the support frame 10.

FIG. 4 shows another manner of causing an untuned circular structure 30to resonate by means of fixing lugs 32 and 34 attached to the structure30. In this specific embodiment two end bars 36 and 38 clamp theelectro-acoustic assembly to the aforementioned fixing lugs 32 and 34.The coupling bars 36 and 38 are tuned to an integer multiple of thehalf-wavelength at the output frequency of the converter 12.

If necessary, the exterior surface of the coupling bars 36 and 38 maynaturally have a profile matching the specific configuration of theframe 30 and compatible with the resonant frequency of the converter 12.

The embodiment shown in FIG. 5 is a variant of the device shown in FIG.3. In this case, however, the electro-acoustic assembly comprising theconverter 12 and the associated two coupling bars 40 and 42 is disposedoutside the operative part of the support frame 10 carrying the filtercloth 20.

In this design the coupling bars 40, 42 may be welded or preferablyscrewed to the converter, the coupling to the extension of the supportframe being advantageously achieved by screwing a tuned nut onto thescrewthreaded end 44 of each coupling bar 40, 42.

Any appropriate rigid fixing means may be used, the essentialrequirement being to achieve good mechanical coupling between the frame10 and the ultrasonic device of the invention. This specific embodiment,in which the ultrasonic device is disposed outside the working area ofthe structure 10, has the advantage that it can be used in a moistatmosphere, for example. Note also that this design enables thestructure or even the combination of the structure and theelectro-acoustic device to be disposed in a sealed chamber with acontrolled gas atmosphere.

FIG. 6 shows an embodiment in which a converter 12 excites a stack ofthree identical structures 10 in the form of filter cloth supportframes. In this type of configuration each structure 10 vibrates in itsown mode and resonates with the ultrasonic emitter device. The couplingof the various structures 10 to the frame of the ultrasonic device isnot rigid. An elastomer type material is advantageously used to fix thestructure and to provide a seal between the stages. Each structure iscoupled to the adjacent structure in the same way in order to preservethe phenomenon of acoustic resonance at each stage. The flexiblecoupling provided by the elastomer-based material provides a sealwithout clamping the various stages of the structure, which aretherefore able to resonate. As in the embodiments previously describedthe converter 12 is attached to fixing lugs 46 by a plurality ofcoupling bars 48 preferably screwed together. Of course, according tothe present invention the fixing members comprising the coupling barsare tuned to half the wavelength of the converter 12.

FIG. 7 shows another embodiment of a device for exciting a stack ofuntuned circular structures 10. The device shown is similar in every wayto that shown in FIG. 6 except that the structures are of circularshape. Finally, FIG. 8 shows a final embodiment in which the vibratordevice comprises an ultrasonic converter 12 screwed to a coupling bar 50tuned to half a wavelength and clamping a circular plate 52 constitutingthe upper disk of a cylindrical drum 54 which is a support member of amesh or filter, for example.

In all the embodiments previously described it may be advantageous toadapt the ultrasonic transmission members to the materials of thestructure to be caused to vibrate acoustically. This can be achieved bymatching the acoustic impedance. This prevents any heating at theconnections and therefore can increase the electro-acoustic efficiencyof the assembly caused to vibrate.

Also to enhance the performance of the ultrasonic device in accordancewith the invention it may be advantageous to use two emissionfrequencies that are not in quadrature, so as to eliminate nodal zonesfrom the working surfaces, on the vibrating cloth, for example. Usingtwo ultrasonic members at frequencies that are not in quadrature, suchas 20 and 30 kHz, for example, nodal zones on the filter cloth, whichare inactive regions, are avoided.

Finally, note that the ultrasonic vibrator device in accordance with theinvention may be used in conjunction with any low-frequency vibratordevice commonly available. The emission of ultrasound communicated tothe untuned structure may be continuous or pulsed. As previouslymentioned, it may be superimposed on low-frequency vibration in therange from 100 to 3,000 vibrations/minute at amplitudes in the order of1 to 30 mm and preferably in a range from 300 to 1,500 vibrations/minuteat amplitudes in the order of 5 to 20 mm.

Depending on the specific application intended, symmetricalbidirectional or asymmetrical unidirectional electro-acoustic convertersare used to emit the ultrasound. Also, the amplitude of the ultrasonicvibrations is matched to the product treated in said structure and isadvantageously between 2 and 30 microns peak-to-peak and preferablybetween 5 and 20 microns peak-to-peak.

What is claimed is:
 1. Device for causing an untuned screening structureto vibrate at an ultrasonic frequency comprising at least oneelectro-acoustic converter adapted to vibrate in a given vibrationdirection and fixed rigidly to said screening structure by metal fixingmembers tuned to the frequency of the converter, the couplings with thestructure being disposed in a region of maximum amplitude of said fixingmembers, the metal fixing members being disposed in said vibrationdirection of the converter and, said structure being untuned at saidultrasonic frequency, the whole is caused to resonate by fixing a nut orany metal assembly tuned to an integer multiple of half the wavelength.2. Ultrasonic device according to claim 1 characterized in that saidmetal fixing members comprise at least one bar (10) coupling to saidstructure (10) the exterior surface of which may be shaped to match anydimensional configuration of said structure whilst remaining compatiblewith the resonant frequency.
 3. Ultrasonic device according to claim 2characterized in that the exterior surface of the coupling bar has inthe vicinity of its nodal zone a radial contraction which has symmetryof revolution about the axis of said bar.
 4. Ultrasonic device accordingto claim 2 characterized in that the exterior surface of said bar has inthe vicinity of its nodal zone a radial enlargement which has symmetryof revolution about the axis of said bar.
 5. Ultrasonic device accordingto claim 1 characterized in that the operating frequency is between 10and 100 kHz and preferably between 20 and 40 kHz.
 6. Ultrasonic deviceaccording to claim 1 characterized in that the ultrasound emission ispulsed or continuous.
 7. Ultrasonic device according to claim 1characterized in that the amplitude of the ultrasonic vibrations isadapted to the product treated in said structure and is advantageouslybetween 2 and 30 microns peak-to-peak and preferably between 5 and 20microns peak-to-peak.
 8. Ultrasonic device according to claim 1characterized in that two emission frequencies not in quadrature areused simultaneously to eliminate nodal zones on the operative surface ofsaid structure.
 9. Ultrasonic device according to claim 1 characterizedin that an ultrasonic converter causes to vibrate and resonate a stackof untuned structures fastened together at the points of connection tosaid electro-acoustic device.
 10. Ultrasonic device according to claim 9characterized in that said untuned structures vibrating and resonatingare disposed in an enclosure adapted to contain a moist atmosphere. 11.Ultrasonic device according to claim 1 characterized in that theelectro-acoustic/structure assembly is disposed in a sealed chamber. 12.Ultrasonic device according to claim 1 characterized in that theemission of ultrasound to the untuned structure is continuous or pulsedand superimposed on low-frequency vibration in the range of frequenciesfrom 100 to 3,000 vibrations/minute for amplitudes in the order of 1 to30 mm and preferably in the range of frequencies from 300 to 1,500vibrations/minute for amplitudes of 5 to 20 mm.
 13. Ultrasonic deviceaccording to claim 1 characterized in that the ultrasound is emitted bysymmetrical bidirectional or asymmetrical unidirectionalelectro-acoustic converters.
 14. Ultrasonic device according to claim 1characterized in that said structure is a screening structure. 15.Device for causing a stack of untuned structure to vibrate and resonateat an ultrasonic frequency comprising one electro-acoustic converteradapted to vibrate in a given vibration direction and fixed rigidly tosaid structure by metal fixing members tuned to the frequency of theconverter, the untuned structures being fastened together at the pointsof connection to said electro-acoustic device, the couplings with thestructure being disposed in a region of maximum amplitude of said fixingmembers, the metal fixing members being disposed in said vibrationdirection of the converter, said structure being untuned at saidultrasonic frequency, the whole is caused to resonate by fixing a nut orany metal assembly tuned to an integer multiple of half the wavelength.